The present invention relates to cooking temperature calibration for electronically-controlled thermal ovens also having pyrolitic self-cleaning capability.
It is well known that cooking ovens require some means for adjusting temperature calibration, particularly through the normal range of cooking temperatures, approximately 150.degree. F. to 550.degree. F. Such calibration capability is required for a number of reasons, including manufacturing tolerances, variability of oven liner form factors and heater wattages in different ovens, and field service due to customer preference.
Pyrolitic self-cleaning ovens operating in accordance with the principles disclosed in the Hurko U.S. Pat. No. 3,121,158 additionally periodically operate at a much higher temperature, for example 880.degree. F., during self-cleaning operation. This particular temperature is factory-calibrated and preset, and normally is not changed in the field. Field change of self-cleaning temperatures is undesirable primarily as a result of safety considerations, but additionally from present regulatory agency requirements.
Recently, various forms of electronically-controlled thermal ovens have been developed, for example utilizing microprocessor-based control systems and triac switching elements to control the required oven functions.
In order to provide temperature feedback for both a relatively lower range of normal cooking temperatures as well as at the relatively higher temperature for pyrolitic self-cleaning, electronically-controlled ovens typically include a single oven-temperature sensor, such as a thermistor, having a known resistance-temperature characteristic. In particular, the temperature sensor for electronic controls is placed in series with a precision voltage divider resistor, and the pair then connected between a fixed DC voltage and a system analog ground. Variation in the sensor resistance due to temperature changes in the oven therefore causes the tap point voltage between the sensor and the voltage divider resistor to vary. This tap point voltage is proportional to oven temperature, and is used by the controller. In an electronically-controlled oven, this voltage is typically converted to digital form by means of a conventional analog-to-digital converter.
An adjustable resistor in series with the sensor could be used to offset the sensor output for calibration purposes. However, not only would this affect calibration through the normal range of cooking temperatures, but it would affect the self-cleaning temperatures as well, an undesirable result.
In any oven temperature adjustable calibration system, an important consideration is ease of adjustment. Particularly for field adjustment procedures, it is desired that adjustments be made as quickly as possible, without involving a cumbersome procedure. For example, a simple set-screw or adjustable potentiometer adjustment may be tedious to use in the field because such arrangements generally do not provide feedback or indication of the amount of adjustment. For example, the field service technician may desire to raise the oven temperature by 25.degree. F., but then has no certain way, absent trial and error, of knowing precisely how much adjustment is required. On thermal ovens which utilize electromechanical temperature controls (hydraulic thermostats or similar devices), one method for adjustment has been to offset the knob which indicates the set point from the calibrated position to a slightly different position. However, on electronic controls which have no knobs, this technique of offsetting the knob set point is impossible.